1. Field of the Invention
The present invention relates to systems for determining the location of a ball striking a court or playing field, and more particularly to systems to determine whether a ball is in or out of play.
2. Description Relative to the Prior Art
Numerous systems for calling lines in tennis and other court games have been devised over the years, but the task is inherently difficult and the performance of such systems has been disappointing. Of the various systems designed for tennis only the "Cyclops" system is in routine use at major tennis tournaments. The Cyclops calls the back service line only and uses light beams crossing the court. As a result it can be used only in singles matches and must be enabled manually before the serve and disabled after the serve to prevent the annoyance of irrelevant service line calls when a ball or player breaks the light beam during play.
To call all lines, all of the time, is a very difficult problem. First of all, line calling requires a very high degree of precision. Professional linesmen are expected to make line calls with a precision of a fraction of an inch while a ball traveling at over 100 miles per hour traverses the width of a court boundary line in a thousandth of a second.
Secondly, systems which attempt to locate ball bounces must avoid being confused by many possible sources of interference. Systems which operate by sensing contact with the court must distinguish between ball bounces and the impact of players' feet and racquets. Systems which operate by tracking the ball must not be distracted by players running between the ball and the sensing device(s).
Thirdly, which of the court boundary lines are relevant changes during play. During the serve, the relevant boundaries are those of one of four service boxes for ball bounces and the opposite baseline for foot faults. But during play, only ball bounces outside the (singles or doubles) court boundaries, double bounces, double hits and net-cord touches by players are relevant. The service boxes are irrelevant as are the positions of players' feet and net-cord touches by the ball.
The capability required in an automatic line calling system depends on the circumstances of its use. At a supervised tournament, such a system can be merely an aid to the chair umpire and line calling staff. The chair umpire, who keeps the score, knows which are the relevant boundaries at each moment of play and can control the system to monitor only those relevant boundaries. This level of manual control is impractical, however, with unsupervised (e.g., club and home court) play. Players can't stop after a successful serve to instruct the line calling system to stop monitoring the service lines and instead to monitor the doubles (say) lines. Consequently, to be applicable to unsupervised play, any line calling system must be able to keep track of the state of play and thus to keep score, without human intervention, except possibly for some rarely occurring situations. Systems for automatically calling lines in tennis matches have been sought after for many years.
Although there are many other games which require a determination of whether a ball is in or out of play, tennis presents a unique problem in that such a determination must be made every time a ball is struck by a player. Furthermore, the ball speeds in tennis, often in excess of 100 miles per hour, make judging lines more difficult than in most other games. In some cases a single call can be the difference between winning and losing, and, in a professional tournament, this could be a difference of several hundred thousand dollars in prize money.
And finally, a thoroughly officiated tennis match requires eleven officials, consisting of ten linesman and the umpire, all of whom have the responsibility of calling lines.
The burden of providing a full complement of officials is great enough that even in the professional ranks games are often played with less than the full complement of officials. In tennis tournaments conducted at regional and local levels, it is common to have the players call their own lines throughout the tournament except during the finals. Even at the club and recreational level the calling of lines presents a source of friction among the players, and negatively effects their enjoyment of the game.
As previously stated, only "Cyclops" is currently in common use, appearing at the highest-level tournaments, including the "Big Four": Wimbledon, the U.S. Open, the French Open, and the Australian Open. Cyclops suffers from a number of shortcomings, however. First of all, it is expensive, being leased for $20,000 per system for two weeks of use. Secondly, Cyclops can only be used to call service lines, and only in singles matches. It cannot distinguish between a ball in flight and a player. Thus, it must be shut off immediately after each serve.
Furthermore, Cyclops is often accused by the players of being inaccurate, and players frequently ask that Cyclops be turned off during a match.
The prior art may be divided into several categories: first, the light-sensor types, such as Cyclops, which has the problem of high inherent cost, requirements of delicate alignment, and which can be rendered inoperable by the presence of players blocking the light beam. Included in this type is Chen, U.S. Pat. No. 4,004,895; Carlton, U.S. Pat. No. 4,867,449 and U.S. Pat. No. 5,303,915.
Next are the pressure-sensitive types, which require pressure-sensitive devices to be embedded in the court, and which generally cannot distinguish between ball bounces and other pressure on the court, including pressure created by the players' movements. Typical of this class is Levine, U.S. Pat. No. 4,365,805.
Then there are the types which require electrically-conductive balls, and which require a network of electrical circuits on the surface of the tennis court itself which sense the presence of the conducting ball which makes circuit connections upon contact. Typical of these is the Van Auken patent, U.S. Pat. No. 4,109,911, and Supran, U.S. Pat. No. 4,432,058.
Finally there are other assorted systems, including the radar systems, such as Nuttall, U.S. Pat. No. 5,138,322, and including the electromagnetic field-sensing systems, such as Candy, U.S. Pat. No. 5,303,915. These are inherently very expensive, and, as in the other types of systems, have difficulty distinguishing between the various types of objects detected.
None of the above systems (with the possible exception of those using pressure sensing) has the slightest possibility of matching the accuracy possible with a clay court where in/out calls can be made (albeit after the fact) by careful inspection of the mark left by the partially compressed and rolling or skidding ball when hitting the court. Further, none (with the possible exception of the radar approach of Nuttall) has the possibility of providing fully automatic operation, because the correct area of the court in which the ball must bounce changes continually in response to events which they don't even attempt to sense, viz., players' racquets hitting the ball.
In contrast to the previous inventions the current invention combines two types of sensors: pressure sensors along the court boundary lines and multiple microphones around the periphery of the court. Whereas, properly designed pressure sensors can provide a very strong signal when hit by a ball and no signal for a miss (even a fraction of a centimeter away), it is difficult or impossible to distinguish (based on the pressure sensor signal alone) whether the sensor signal was caused by a ball hit or some other event or by even electrical interference.
On the other hand, the sound of a ball bounce as picked up by a microphone is very distinctive and by measuring the time of arrival of this distinctive sound at a multiplicity of microphones, a good--but not precise--estimate of the location of the ball bounce and, equally importantly, of the time of the bounce (to within a few milliseconds) can be made. When the location of the ball bounce is not very close to a court boundary, the microphone signals alone can be used to determine if the ball was in or out, but if the bounce is very close to the boundary line, the pressure sensor signals are also used but only during the brief instant of time when (based on the estimated time of the bounce) the ball might have hit the pressure sensor. Thus the problem of excluding irrelevant signals from the pressure sensors is solved, not by analysis of the pressure signal itself, but rather by inspecting the pressure sensor signals at such short and infrequent times that the probability of a coincident interfering signal is negligibly small.
The current invention solves all of the problems inherent in the prior art. First of all, it is inherently inexpensive to produce. It utilizes commonly available products: coaxial cables to serve as pressure transducers, commercially available microphones as sound detectors, and commercially available loudspeakers as sound generators and alarms. It should be noted, however, that the term "coaxial cable" is used here to indicate a cable comprising a conductive outer shield, a dielectric, and one or more center conductors surrounded by the dielectric. The outer shield likewise completely surrounds the dielectric. Coaxial cables, as used in the context of this invention, include not only the commonly used type whose cross section comprises concentric circles, but flattened cables having a width of several inches, and a thickness measured in thousandths of an inch.
Only a single, custom-designed circuit board together with a computer interface board is required to allow input of the signals from the microphones and pressure transducers into a personal computer, and to allow the personal computer to generate signals to drive the loudspeakers.
Furthermore, the current invention is able to distinguish among ball bounces on the court, balls striking the net, balls hitting racquets, and to overlook players moving around the court and other background sounds.
The current system is equally applicable to the hard surfaces currently popular, including asphalt, concrete, and composition courts, to carpet-like surfaces, as well as to granular surfaces, such as clay, Har-Tru, terre-battue, and other related materials.
Because the current system is able to accurately locate the position of the ball as it strikes the court each time and to detect and recognize other relevant events, such as a player's racquet hitting the ball, the system provides for automatic scoring capability.
And finally, the current invention provides for statistical collection of ball bounces, location of racquet hits, ball speed, etc. to be used as a training aid.
According to one aspect of the current invention, coaxial cables are placed in intimate contact with the several boundary lines of the tennis court. These cables may be buried in the court beneath the boundary lines, or may be attached to the lines themselves, where the lines are provided in rolls of plastic or canvas materials.
According to yet another aspect of the invention, coaxial cable pressure sensing devices may be placed just outside of the court boundary lines to increase the area of the court instrumented with pressure sensing devices and thus to increase the reliability of in/out decisions.
According to another aspect of the invention, a multiplicity of microphones is provided around the periphery of the tennis court. These microphones sense sounds caused by the hitting of the tennis ball with a variety of objects, as well as with the ground.
According to still another aspect of the invention, the electrical signals produced by the microphones are analyzed and filtered so that only ball bounces on the court, racquet hits and net-cord hits are provided to the computation system.
According to yet another aspect of the invention, the time delays of signals arriving at several pairs of microphones and arising due to a ball bounce, racquet hit or net-cord hit are accurately determined, and these time delays are processed by a computation device to precisely locate the ball anywhere on the court or proximate surrounding area to within an error less than the width of the standard court boundary lines.
According to yet another aspect of the invention, a multiplicity of loudspeakers is provided directly adjacent to the microphones to provide for calibration of the sound system to take into account variations in transmission time of sounds through the air due to temperature and wind conditions.
According to yet another aspect of the invention, the output of the coaxial cables is considered only when the sound system calculates that the ball has hit the court very close to one of the boundary lines so that false indications of ball contact cannot be induced by players' feet and other disturbances except by coincidence during very brief instants of time.
According to another aspect of the invention, the pressure sensing devices are adjusted to have a contact sensitivity closely approximating the sensitivity of a clay court to marking by an impacting ball.
According to another aspect of the invention, the approximate location provided by the microphone information is used together with the precise contact information provided by the contact sensors to make an automatic determination in real time and to a precision equivalent to checking the mark on a clay court as to whether each ball bounce is in or out.
According to yet another aspect of the invention, contact with the net by a ball or a player is sensed by a pressure sensitive device or devices attached to the net.
According to yet another aspect of the invention, foot faults are detected by comparing a) the time of occurrence of signals induced by contact of a serving player's foot acting on a pressure sensor associated with the court baseline with b) the computed time of occurrence of racquet contact with the ball as derived from the racquet sounds received by three or more microphones.
According to yet another aspect of the invention, the sequence of locations of ball bounces, net-cord hits, service foot faults and racquet hits (the first three of which may be confirmed using signals from the pressure sensing devices) and resulting in/out calls are used to monitor the progress of play and to keep score.
According to a final aspect of the invention, the sequence of precise times and locations of ball bounces, net-cord hits, racquet hits and service foot faults is stored and used to provide replay of the match and/or to compile statistics of player performance such as ball speed (both on serves and otherwise), numbers of winners (unreturned hits), errors (hits which go out) and other measures of player accuracy and effectiveness.